The present invention is directed to a chiral nematic liquid crystal material for use in a display device. More specifically, the invention relates to chiral nematic liquid crystal materials that include organic additives in an amount effective to avoid image sticking in a liquid crystal display device.
Current cholesteric liquid crystal display devices, upon application of an electric field, exhibit a planar texture which selectively reflects a certain wavelength of light depending on the pitch length of the material. A focal conic texture is also present, in which light is scattered, not reflected. Such displays are preferably designed so that both the focal conic and planar textures are stable in the absence of an electric field.
Cholesteric displays are often used in public information signs and static image applications because of the wide viewing angle and its low power bistable operation modes. During operation of the liquid crystal display device, there are times when a pixel within the display remains in the focal-conic texture for some period and is then switched to the planar texture and vice versa. If the intensity level of this planar state is less than that of a pixel that was previously in the planar state, or conversely, if the intensity levels between focal conic textures of the same pixel are greater than that of a pixel that was previously observed in the earlier focal conic texture, the display is said to have image sticking. As a result of image sticking, the previous image of the display affects the new image of the display: a ghost image from the previous image appears in addition to the new image. For example, suppose the capital letter xe2x80x9cTxe2x80x9d is displayed by a cholesteric liquid crystal display device. The display may comprise a dark background wherein the light is scattered by the focal conic texture. The letter xe2x80x9cTxe2x80x9d would be defined by a brighter area within the dark background in which the liquid crystal is in the planar texture. Now suppose the letter is changed to the capital letter xe2x80x9cIxe2x80x9d and superimposes the position where the letter xe2x80x9cTxe2x80x9d was observed. If image sticking occurred, the outwardly extending horizontal portions of the letter xe2x80x9cTxe2x80x9d would still be slightly visible. In this example, the horizontal portions would be slightly lighter than the surrounding dark background but darker than the light portion used to display the letter xe2x80x9cIxe2x80x9d. Thus, the display would undesirably show a ghost image of the non-overlapping portions of the letter xe2x80x9cTxe2x80x9d. Generally, between updates, a display is refreshed to minimize the effect of image sticking. However, image sticking can be strong enough so that a refresh pulse is unable to completely erase the previous image.
Efforts have been made to eliminate image sticking or reduce the degree of image sticking by modification of driving waveforms. Various types of erasing waveform combinations have been discussed previously. For example, U.S. Pat. No. 5,644,330 entitled xe2x80x9cDriving Method for Polymer Stabilized and Polymer Free Liquid Crystal Displaysxe2x80x9d, to Catchpole et al. discloses erasing waveforms. By employing a good erasing waveform, image sticking can be significantly reduced or even eliminated in some cases. However, it is noticed that in certain display configurations, image sticking cannot be completely removed solely by optimization of the driving waveforms. The degree of image sticking becomes very profound when the time duration between updates is long, i.e., greater than 24 hours. End users can usually heat the display above the isotropic transition temperature to eliminate the image sticking result from long time storage or shipping. However, this practice is not cost effective and user friendly. The invention addresses the problem of image sticking from a liquid crystal formulation approach.
The present invention is directed in general to a liquid crystal mixture for a display device which comprises a first chiral nematic liquid crystal component, and a second component that, by itself, exhibits no liquid crystalline phase at any temperature. The second component is present in an amount effective to avoid image sticking and/or reduce the bulk viscosity of the material by at least about 26% and, more preferably, by at least about 46%. The liquid crystal mixture preferably has a positive dielectric anisotropy. Display devices that include the present material exhibit greatly improved properties including higher contrast ratios, shorter pulse widths, lower driving voltages and no image sticking during display use.
In particular, at least in the case of viscosity lowering additives, the second component is achiral and has a molecular weight not exceeding about 205 grams/mole. The second component, when used as an image sticking additive, is preferably in an amount ranging greater than 0% to not greater than 5% by weight based upon the total weight of the material and, more preferably, in an amount ranging up to 3% by weight. Amounts of all chemical components herein are in % by weight based upon the total weight of the liquid crystal material, unless otherwise indicated.
A preferred embodiment of the invention is directed to a display device comprising a liquid crystal material including the first chiral nematic liquid crystal component, and the second component which exhibits no liquid crystalline phase at any temperature. The second component is effective for avoiding image sticking during display use and/or reducing viscosity of the liquid crystal material. Cell wall structure cooperates with the material to form focal conic and twisted planar textures that are stable in the absence of a field. A device applies an electric field for transforming at least a portion of the material to at least one of the focal conic and twisted planar textures.
The first component is a chiral nematic liquid crystal material. The second component may be a blend of additives that give the desired properties. More preferably, the second component is one compound that avoids image sticking. The material cooperates with the cell wall structure to form focal conic and twisted planar textures that are stable in the absence of a field.
The second component that helps avoid image sticking and/or lowers the viscosity of the material may be comprised of a compound having the following general formula I: 
wherein R1 is selected from the group consisting of an alkyl group, a substituted alkyl group and an alkoxy group each having from 1 to 9 carbon atoms.
More preferably, the second component may have the following formula: 
wherein A comprises any group that renders the second component polar and soluble in the first component. In this regard, group A preferably comprises a cyano group. In the case wherein the additive is used for viscosity lowering properties, when group A is a cyano group the second component preferably does not include seven carbon atoms.
Preferred substituted phenyl compounds that satisfy the general formula I comprise the following: 1-phenylpentane, 1-phenylhexane, 1-cyclohexylbenzene, p-cyanopentylbenzene, 4-phenylbutyronitrile and p-cyanophenylheptane.
Alternatively, the second component for avoiding image sticking and/or lowering the viscosity of the material may be comprised of a compound having the following formula II:
R2xe2x80x94Bxe2x80x83xe2x80x83II.
wherein R2 is selected from the group consisting of an alkyl group and an alkoxy group each having from 1 to 11 carbon atoms and B comprises any group that renders the second component polar and soluble in the first chiral nematic liquid crystal component. A preferred second component having a formula that satisfies formula II is undecyl cyanide.
The second component in the case of both formulas I and II preferably has a molecular weight not exceeding about 205 grams/mole at least in the case of viscosity lowering additives and possibly for additives that avoid image sticking. Groups A and B are preferably cyano groups. R1 and R2 are preferably alkyl, substituted alkyl groups or alkoxy groups.
The image sticking and/or viscosity lowering additive of formula I is preferably comprised of a monocyclic aromatic compound such as benzene. However, those skilled in the art would appreciate in view of this disclosure that the viscosity lowering and/or image sticking additive may comprise other aromatic compounds. For example, the image sticking and/or viscosity lowering additive may comprise a heterocyclic compound. In this regard, nitrogen heterocyclic compounds (e.g., pyridine) may be suitable for use in the present invention. Biphenyl compounds are generally not suitable for use in the present invention unless they do not exhibit a liquid crystalline phase at any temperature and have a molecular weight less than about 205 grams/mole. It is well within the abilities of a skilled chemist to empirically determine which aromatic or heterocyclic compounds are suitable for use in the present invention by using these compounds in place of the benzene ring in accordance with formula I, adding the resultant material to a chiral nematic component and observing whether the material is soluble and avoids image sticking during display use and/or lowers the viscosity of the material according to the present invention.
The following is a list of definitions of terms used in this disclosure:
xe2x80x9cHeteroatomxe2x80x9d is a nitrogen, sulfur or oxygen atom. The heteroatoms of groups that contain more than one heteroatom may be the same or different.
xe2x80x9cAlkylxe2x80x9d is an unsubstituted or substituted saturated hydrocarbon chain radical. Preferred alkyl groups have from 1 to 11 carbon atoms (xe2x80x9clower alkylsxe2x80x9d) and include (for example) methyl, ethyl, propyl, butyl, pentyl and heptyl.
xe2x80x9cCycloalkylxe2x80x9d is a saturated hydrocarbon ring radical. One preferred cycloalkyl is cyclohexyl.
xe2x80x9cHeterocyclic ringxe2x80x9d is an unsubstituted or substituted, aromatic ring radical comprised of carbon atoms and one or more heteroatoms in the ring. The aromatic compounds of the invention, including any heterocyclic compounds, are preferably monocyclic.
xe2x80x9cAlkoxyxe2x80x9d is an oxygen radical having a hydrocarbon chain substituent, where the hydrocarbon chain is an alkyl or alkenyl (ie., xe2x80x94O-alkyl or xe2x80x94O-alkenyl).
As defined above and as used herein, substituent groups may themselves be substituted. Substituents that may be suitable for use in the present invention are listed in C. Hansch and A. Leo, Substituent Constants for Correlation Analysis in Chemistry and Biology (1979), which is incorporated herein by reference.
These and other objects, advantages and features of the invention will become better understood from the detailed description of the invention which is described in conjunction with the accompanying drawings.